This invention relates to coated silicone rubber articles, and more particularly to coatings of fluorocarbon elastomers that impart abrasion resistance, a reduced level of gas permeability and other desirable properties to silicone rubber substrates.
The prior art discloses coating silicone rubber with various materials for the purpose of imparting desirable properties or eliminating undesirable properties inherent in the rubber. U.S. Pat. No. 3,639,155, issued to R. C. Hartlein and H. L. Vincent on Feb. 1, 1972 teaches that the tendency of silicone rubbers to accumulate tenaciously adhering dirt particles from the surrounding environment can be eliminated by coating the rubber with a room temperature vulcanizable block copolymer containing three specified types of organosiloxane repeating units. In addition to preserving the initially attractive appearance of the silicone rubber, many of the cured coatings are more resistant to abrasion and degradation by certain chemical agents, particularly strong acids and bases, than the underlying rubber.
The coatings disclosed in the aforementioned Hartlein et al. patent are less than satisfactory for some applications, for example those during which a silicone rubber is exposed to organic solvents, particularly strong acids and bases, or when it is desired to reduce the permeability of silicone rubber to oxygen or other gases.
Of the numerous types of organic polymers that have been used as coating materials for various metallic and nonmetallic substrates, those derived from ethylenically unsaturated fluorocarbons such as tetrafluoroethylene, vinylidene fluoride and hexafluoropropylene have been shown to be among the most resistant to abrasion and destruction by chemical agents. In addition, the vapor permeability of these polymers is relatively low compared to polyorganosiloxanes. Another inherent characteristic of fluorocarbon polymers, including elastomers, derived from vinylidene fluoride and/or hexafluoropropylene, is their resistance to "wetting" by water, most organic solvents, and adhesives of almost every type. These physical and chemical properties of fluorocarbon elastomers, also referred to as fluoroelastomers, make them ideal candidates for protective coatings on silicone rubber articles requiring oxidative stability, resistance to high temperature abrasion, and/or chemicals which dissolve or degrade many other types of synthetic organic polymers.
Since about 1950 numerous ethylenically unsaturated fluorine-containing hydrocarbons have been incorporated into elastomeric polymers. Elastomers derived from many of these hydrocarbons, including vinylidene fluoride, hexafluoropropylene and perfluoroalkyl vinyl ethers are now commercially available from a number of suppliers. A detailed discussion of the various types of fluoroelastomers is contained in an article by R. G. Arnold, A. L. Barney and D. C. Thompson that appeared in the July, 1973 issue of a journal entitled "Rubber Chemistry and Technology" (Volume 46, pp. 619-652). A more recent article by R. A. Brullo and R. R. Campbell (Elastomers, April 1980, pp. 47-51) discloses typical fluorocarbon polymer formulations that can be cured to form elastomer products.
The polymer art employs the terms "curing," "vulcanization" and "crosslinking" to define a process during which molecules of a thermoplastic polymer, including fluoroelastomer gums, are joined together by chemical bonding to form a nonthermoplastic material. A variety of methods for curing or crosslinking fluoroelastomer gums have been employed, including reaction with peroxides, diamines, dithiols, and aromatic polyhydroxy compounds, and the use of radiation. The aforementioned article by Arnold et al. reports that all of these curing systems require the presence in the formulation of a basic metal oxide such as magnesium or calcium oxide. The basic metal oxide reacts with the acidic by-products, which are believed to include hydrogen fluoride and/or derivatives thereof, that are generated during curing (crosslinking) of the elastomer. In the absence of the basic metal oxide the by-products of the curing reaction would eventually react with the polymer and result in a gradual degradation of desirable physical properties such as hardness and tensile strength. While the basic metal oxides currently employed in fluoroelastomer gums are necessary to react with these by-products and thereby preserve the desirable physical properties of the cured elastomer, the oxides function as nonreinforcing fillers. It is known that such fillers detract from the physical properties of the unmodified elastomer. In addition, the basic metal oxide particles render the elastomer opaque and unsuitable for end use applications requiring optically clear materials.
U.S. Pat. No. 4,323,603, issued on Apr. 6, 1982 to David Close discloses replacing the basic metal oxides conventionally used as acid acceptors in conventional curable fluoroelastomer compositions with a silane of the general formula EQU RCH.sub.2 CH.sub.2 CH.sub.2 SiR'.sub.3-n (OR").sub.n
wherein R is a "reactive organofunction group such as amino or epoxy," R' is alkyl containing from 1 to 5 carbon atoms and R" is alkyl containing from 1 to 4 carbon atoms. This patent teaches that cured fluoroelastomer compositions containing the aforementioned silanes adhere better to various substrates, including metal, glass and rubber, than prior art formulations containing basic metal oxides. Experimental data in this patent demonstrate that while a cured fluoroelastomer composition containing an epoxypropyl silane (identified as Z-6040, a product of the Dow Corning Corporation) in place of the basic metal oxide exhibited good to excellent adhesion when applied to aluminum and glass, the adhesion to a nitrile- or neoprene-based rubber was considered only fair, in that the coating could be peeled away from the substrate. This patent also teaches using the silane as a primer layer over which a solubilized fluoroelastomer gum is applied.
Up until the present invention, direct cohesive bonding between a fluoroelastomer and a silicon rubber in the absence of an intervening primer or adhesive layer has not been considered practical because of the inherent inability of both fluorocarbon polymers and silicone rubber to strongly adhere to other surfaces. The prior art teaches that a variety of materials can be used as primers, adhesives or modifiers to achieve bonding between a silicone rubber and fluorine-containing organic polymers. U.S. Pat. No. 3,945,773 to A. Panagrossi et al., issued on July 19, 1960, discloses employing a finely divided mixture of a solid fluorinated polymer and a solid natural or silicone rubber as an adhesive for bonding a layer of fluorinated polymer to a substrate of a dissimilar material, such as a silicone rubber. A layer of finely divided rubber particles is placed between the aforementioned adhesive mixture and the solid rubber substrate.
The prior art teaches the addition of various materials to an uncured silicone rubber formulation in order to improve the adhesion between the cured rubber and other surfaces. For example, U.S. Pat. No. 4,082,726 to Mine et al., issued on Apr. 4, 1978, discloses an uncured organopolysiloxane composition containing (1) an organopolysiloxane having at least two lower alkenyl groups per molecule, (2) an organopolysiloxane having at least two silicon-bonded hydrogen atoms per molecule, (3) an organopolysiloxane having at least one epoxide group per molecule and (4) a suitable catalyst. Mine et al. teach that such compositions can be applied to a variety of substrates and subsequently cured to yield adherent coatings. The teaching of this reference would not be applicable to bonding a previously cured silicone rubber to other substrates or to bonding uncured rubbers that are curable by means other than the reaction of ethylenically unsaturated hydrocarbon groups with silicon-bonded hydrogen atoms.
The use of primer compositions containing silanes or polysiloxanes having epoxy and/or vinyl groups in the molecule for the purpose of adhering silicone rubbers to various substrates is disclosed in Japanese Patent Publication No. 3,127/79, published on Jan. 11, 1979, U.S. Pat. No. 3,619,256, issued on Nov. 9, 1971, U.S. Pat. No. 4,332,844 to Hamada et al., issued on June 1, 1982 and U.S. Pat. No. 4,122,127 to Mikami et al., which issued on Oct. 24, 1978. Such primer coatings are undesirable, since they require additional steps in the process for preparing coated silicone rubber articles. Many coatings require the use of organic solvents which must subsequently be recycled or disposed of.
British Pat. No. 1,092,857 teaches applying aqueous solutions or dispersions of hydrolyzed alkoxysilanes containing at least one epoxy group to the surface of a silicone rubber substrate to achieve bonding between the rubber substrate and a substrate formed from a polyamide or polyester resin. This method requires hydrolyzing an alkoxy-substituted epoxyalkyl silane, dissolving or dispersing the hydrolyzate in water and applying the resultant composition to the surface of a silicone rubber article prior to contacting the rubber with the resin.
One objective of this invention is to provide silicone rubber substrates coated with a cured fluoroelastomer, wherein the coating and substrate are cohesively bonded in the absence of an intervening layer of primer or adhesive.
A second objective of this invention is to provide a method for adhering a peroxide-curable fluoroelastomer gum to a cured or peroxide-curable silicone rubber without the use of a separate primer or adhesive as taught in the prior art.